Attempt to use AF_UNIX sockets instead of AF_INET on Windows 10+

[dlenski]

This PR is based on the improvements we made in OpenConnect in https://gitlab.com/openconnect/openconnect/-/merge_requests/320.

Unfortunately, and maddeningly, it's possible for the local IPv4 routes (127.0.0.0/8) to be deleted on Windows; this will prevent dumb_socketpair() from working in its current form. Using AF_UNIX sockets

• AF_UNIX/SOCK_STREAM became available in Windows 10: https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows
• There appears to be no other documentation whatsoever of Windows's support for AF_UNIX sockets.
• Contrary to the claims of that blog post, abstract sockets (filesystem-independent) are not implemented: AF_UNIX abstract on Windows do not work microsoft/WSL#4240 (comment)

In order to deal with the above weaknesses, the approach taken here is to first try creating an AF_UNIX socketpair, and then to fallback to AF_INET if it fails. Furthermore, because the AF_UNIX socketpair requires a real, writable filesystem path, it tries binding to each of the following before giving up on AF_UNIX:

1. GetTempDir() \ (filename including PID and 64-bit time ticks)
2. GetWindowsDirectory() \ Temp \ (...)
3. C:\TEMP \ (...)
4. . \ (...)

Based on testing so far, this implementation appears to be robust and to avoid the problems with deleted IPv4 localhost routes.

Also adds a stub for the <afunix.h>, which defines UNIX_PATH_MAX and struct sockaddr_un. MinGW lacks this header, but other FLOSS projects show how to embed the needed definitions:

• MisterDA/ocaml@5855ce5
• https://github.com/curl/curl/blob/curl-7_74_0/lib/config-win32.h#L725-L734